4E-BP1, a Repressor of MRNA Translation, is Phosphorylated and Inactivated by the Akt(PKB) Signaling Pathway

Growth factors and hormones activate protein translation by phosphorylation and inactivation of the translational repressors, the eIF4E-binding proteins (4E-BPs), through a wortmannin- and rapamycin-sensitive signaling pathway. The mechanism by which signals emanating from extracellular signals lead to phosphorylation of 4E-BPs is not well understood. Here we demonstrate that the activity of the serine/threonine kinase Akt/PKB is required in a signaling cascade that leads to phosphorylation and inactivation of 4E-BP1. PI 3-kinase elicits the phosphorylation of 4E-BP1 in a wortmannin- and rapamycin-sensitive manner, whereas activated Akt-mediated phosphorylation of 4E-BP1 is wortmannin resistant but rapamycin sensitive. A dominant negative mutant of Akt blocks insulin-mediated phosphorylation of 4E-BP1, indicating that Akt is required for the in vivo phosphorylation of 4E-BP1. Importantly, an activated Akt induces phosphorylation of 4E-BP1 on the same sites that are phosphorylated upon serum stimulation. Similar to what has been observed with serum and growth factors, phosphorylation of 4E-BP1 by Akt inhibits the interaction between 4E-BP1 and eIF-4E. Furthermore, phosphorylation of 4E-BP1 by Akt requires the activity of FRAP/mTOR. FRAP/mTOR may lie downstream of Akt in this signaling cascade. These results demonstrate that the PI 3-kinase-Akt signaling pathway, in concert with FRAP/mTOR, induces the phosphorylation of 4E-BP1.